RNA interference (RNAi) describes a phenomenon in which the presence of double-stranded RNA (dsRNA) having a sequence that is identical or highly similar to a portion of a target gene results in the degradation of messenger RNA (mRNA) transcribed from that targeted gene (Sharp 2001). Fjose et al. have proposed a mechanism for RNA interference [Fjose et al. RNA Interference: Mechanisms and Applications. Biotechnology Annual Review, Vol. 7, pp. 10-57 (2001)]. Initially a double stranded RNA sequence (dsRNA) sequence is made available with one strand that is identical or highly similar to a target gene and complementary to an mRNA produced from the transcription of the target gene (the sense strand), and an antisense strand that is complementary to the sense strand. Thus, the antisense sense strand has an identical or highly similar sequence to a portion of the mRNA that results from the transcription of the target gene.
An RNAi nuclease then cleaves the dsRNA into short double stranded fragments whose lengths may vary from 18-25 nucleotides, and binds to the mRNA produced from the transcription of the target gene. An RNAi enzyme having helicase activity then catalyzes an exchange between the short dsRNA and the mRNA so that the antisense strand of the dsRNA anneals to the mRNA, replaces the “antisense” strand of the dsRNA, and the mRNA is cleaved at its ends. Consequently, the mRNA is destroyed, and translation of the mRNA molecule does not occur. Moreover, the sense strand of the short dsRNA, which remains bound to the RNAi nuclease, serves as a template for production of a new antisense strand, forming a new dsRNA molecule for use in the destruction of another mRNA produced from the transcription of the target gene. Thus, RNAi demonstrates a catalytic activity. (Id.)
The ability to specifically knock-down expression of a target gene by RNAi has obvious benefits. For example, RNAi may be used to generate animals that mimic true genetic “knockout” animals to study gene function. In addition, RNAi may be useful in treating diseases or disorders that arise from the abnormal expression of a particular gene or group of genes, or the expression of a gene having a particular mutation or polymorphism. For example, genes contributing to a cancerous state (e.g., oncogenes) may be inhibited. In addition, viral genes may be inhibited, as well as mutant genes causing genetic diseases such as myotonic dystrophy, cystic fibrosis, Alzheimer's Disease, Parkinson's Disease, etc. Inhibiting such genes as cyclooxygenase or cytokines may also have applications in treating inflammatory diseases such as arthritis.
Accordingly, what is needed is a vehicle that delivers heterologous RNA into a cell in order to utilize interference RNA to modulate, and particularly, to down-regulate the expression of a particular target gene within a cell.
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